Mechthildis’s father was a grandson of Archduke Charles, Duke of Teschen who had led the Austrian army against Napoleon Bonaparte.

Her father had followed a career in the Austrian Navy and Mechthildis spent her formative years primarily in Istria in the then-Austrian port of Pula in the Adriatic.

Archduke Charles Stephen put aside his career in the navy and centered his ambitions in creating a Polish branch of the house of Habsburg.

The marriage took place on 11 January 1913 at Saybusch Castle, two days after her eldest sister Archduchess Eleonora married Alfons von Kloss.

At the outbreak of World War II they fled to South America and stayed initially at Petropolis with members of the Brazilian imperial family, the Orléans-Braganza.